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We investigate the properties of an $N$-site tight-binding lattice withperiodic boundary condition (PBC) in the presence of a pair of gain and lossimpurities $\pm i\gamma$, and two tunneling amplitudes $t_0,t_b$ that areconstant along the two paths that connect them. We show that the parity andtime-reversal ($\mP\mT$)-symmetric phase of the lattice with PBC is robust,insensitive to the distance between the impurities, and that the criticalimpurity strength for PT-symmetry breaking is given by $\gamma_{PT}=|t_0-t_b|$.We study the time-evolution of a typical wave packet, initially localized on asingle site, across the PT-symmetric phase boundary. We find that it acquireschirality with increasing $\gamma$, and the chirality reaches a universalmaximum value at the threshold, $\gamma=\gamma_{PT}$, irrespective of theinitial location of the wave packet or the lattice parameters. Our resultsimply that PT-symmetry breaking on a lattice with PBC has consequences thathave no counterpart in open chains.

PT-symmetry breaking and universal chirality in a PT-symmetric ring